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Thread: RCDs

  1. #1
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    Default RCDs

    OK, I've bought some C-BUS kit and one thing that I cant seem to ind in the documentation is whether or not you should put an RCD in line with the load on a dimmer unit.

    I know that I will need an 8A RCD for the feed to the dimming unit itsel but is this sufficient to cut the power if one of the load circuits shorts?

    Probably a simple Q but better safe than sorry.
    Ric Charlton

    always trying but not always successful

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    If possible, I would recommend having a RCD in series with each output channel before the load. Without the output RCD protection, it is possible that the dimmer channel could be damaged by the short-circuit condition.
    Kwong Li
    Laser Business Systems Ltd.
    http://www.laser.com
    http://www.cbus-shop.com

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    Default Re: RCDs

    Thanks for the reply!

    This does move me on quite nicely to my next couple of questions:
    • What size MCB should I place in line? - i understand that each channel is a max. of 1A

    • Where can I find 8A MCBs? - or should I use a size that I can get? (6A or 10A)
    Ric Charlton

    always trying but not always successful

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    Default Re: RCDs

    Quote Originally Posted by Ric
    Thanks for the reply!

    This does move me on quite nicely to my next couple of questions:
    • What size MCB should I place in line? - i understand that each channel is a max. of 1A

    • Where can I find 8A MCBs? - or should I use a size that I can get? (6A or 10A)
    Normally circuit protection should be in front of the dimmer not after. If the dimmer is an L5508D1A for example then a 10amp RCDMCB should be used. The chance of a fault taking out the group leg of the dimmer is remote at best. If this does occur you can have your dimmer repaired by Clipsal for a nominal fee.

    The concept of RCD protection on a group by group as suggested by CBusShop is not recommended practice.

    A short circuit condition does not necessarily mean you are going to toast that particular channel either. Of the hundreds of dimmers we have fitted, and yes I do mean hundreds, I can think of only one instance where a short circuit caused a group to fail.

    If you build some redundancy into your project (ie; have a couple of spare groups left on a dimmer as well as a relay) you can also just move the output of that circuit to a new group output and reprogram.

    Hope that helps.

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    The question that I was trying to answer was "I know that I will need an 8A RCD for the feed to the dimming unit itself but is this sufficient to cut the power if one of the load circuits shorts?". I was not advocating not having circuit protection on the input mains feed but rather, in addition of.

    Having RCD protection after as well as before the dimmer, although not mandatory, is recommended practice by Clipsal UK and I have known at least one instance of a short-circuit taking out the dimmer channel in far fewer dimmers than Ross has fitted. Having a dimmer repaired might be a cost-effective option in Australia but not in the UK because Clipsal UK do not have a repairs department at the moment and unlikely to for some time.

    Apart from the small additional cost and space involved, there is no disadvantage in having circuit protection fitted to each output dimmer channel. Indeed, having output channels individually protected as well as the incoming mains feed would allow maximum flexibility in isolating either the whole dimmer or channel by channel basis.
    Kwong Li
    Laser Business Systems Ltd.
    http://www.laser.com
    http://www.cbus-shop.com

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    Automated Home Guru FrankMcAlinden's Avatar
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    Default Re: RCDs

    Hi Ric
    As Ross has mentioned in a previous post its not normal practice here in Oz to put circuit breakers or rcds on the cbus loads. If i was going to put protection on each output i would use a hrc fuse which is much quicker than a normal domestic circuit breaker... I had one channel of one of my dimmers go down and Clipsal repaired it in a resonable time..
    I would imagine as Ross stated that the dimmer units would be quite robust...I do recall someone mentioning that there are now circuit breakers available which are fast acting like hrc fuses ....

    HTH
    Frank

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    Default

    Quote Originally Posted by CBusShop
    The question that I was trying to answer was "I know that I will need an 8A RCD for the feed to the dimming unit itself but is this sufficient to cut the power if one of the load circuits shorts?". I was not advocating not having circuit protection on the input mains feed but rather, in addition of.
    The answer to that question is Yes. RCD and MCB protection is sufficient to protect the dimmer. Circuit protection on the input mains side via MCB is manditory not optional. RCD on the input side could be optional in the UK however in Australia all lighting circuits require and RCD as well. An RCD on the input side will offer greater protection. The point is the upstream RCD will see the short before the downstream RCD every single time. Why double up? Only one will trip and it will be the upstream unit.
    Having RCD protection after as well as before the dimmer, although not mandatory, is recommended practice by Clipsal UK
    Do you have a reference for this statement? Clipsal Australia do not have this as recommend practice. I would be very suprised if CIS UK did. The only reference to circuit protection I have every found is on the line side of the dimmer. The installation data sheet for an 8 channel dimmer can be found here http://www.clipsal.com/cis/pdf_files...NSTRUCTION.pdf and nothing about load protection is mentioned or recommended.
    Having a dimmer repaired might be a cost-effective option in Australia but not in the UK because Clipsal UK do not have a repairs department at the moment and unlikely to for some time.
    That may well be the case and I accept that fully. That should further the cause to have some redundancy built into the design of the project.

    Apart from the small additional cost and space involved, there is no disadvantage in having circuit protection fitted to each output dimmer channel.
    Can you give me a figure in pounds as to the cost of having this protection? I would be interested to hear an amount quoted. My experience says more cost, more space, more time, no gain.
    Indeed, having output channels individually protected as well as the incoming mains feed would allow maximum flexibility in isolating either the whole dimmer or channel by channel basis.
    This is hardly worth mentioning. Isolation of channels from the dimmer is a simple affair and can be achieved with a screw driver and a connector.

    Well thats my 0.02c worth. I'll let you chew on that for a bit while you find me this reference from CIS UK.

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    You have an interesting style of debate, Ross. I shall answer your points in the same order.

    The upstream MCB will not protect a dimmer from a fault condition downstream. By the time the upstream MCB trips due to a fault downstream, the dimmer channel could already have been damaged. By fitting a downstream MCB (as well as the upstream one, of course), it should protect the channel from over-current and short-circuit fault conditions. It also allows easy isolation of a particular dimmer channel to carry out work on. As you are aware, C-Bus dimmer channels are not isolated from the mains feed even in the off state.

    There is no written reference to my statement except that it is endorsed by the CIS UK Technical Manager, Steve Gordon. Also, on a number of CIS quotations to customers that we have copies of, the lack of any downstream channel circuit protection was explicitly stated. Steve has advised that any damage caused by inadequate circuit protection on both sides of the dimmer is not covered by CIS standard warranty. He further confirmed that damaged dimmers are currently not repairable by CIS UK for the foreseeable future.

    If a customer has 8 circuits to control with a C-Bus 8-channel dimmer, I hope you are not suggesting that he or she should install an additional dimmer module as redundancy. A protective MCB for the purpose costs less than 3 British Pounds per channel and a suitable DIN rail enclosure costs around 35 Pounds (all plus VAT) if additional rail space is required.

    May be it is not worth mentioning the quick circuit isolation feature of having downstream MCBs for electricians and people that are electrically competent. For the rest of us, poking around inside a live DIN rail enclosure with a screwdriver rearranging potentially live wires is hardly a simple or desirable activity. Swapping channels and reprogramming the C-Bus dimmer unit, for most people, involves a chargeable visit by the installer/integrator whereas an MCB will provide the protection automatically and conveniently.

    The C-Bus dimmers, as you have stated, are good units but they are not infallible. Dimmer channels do get damaged under certain fault conditions and the occurrence is higher that you are experiencing. For the sake of a few extra Pounds per channel, I maintain that fitting appropriate dimmer channel circuit protection is a sensible and cost-effective precaution.
    Kwong Li
    Laser Business Systems Ltd.
    http://www.laser.com
    http://www.cbus-shop.com

  9. #9
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    Default Re: RCDs

    OK - so, if I go with the protection on either side of the dimmer, what size MCB should I be looking at for the load?

    Each channel has a 1A rating and the smallest standard-size MCB I have seen is a 3A. This would therefore allow 3 times the rated current flow before tripping.

    Ideas? :roll:
    Ric Charlton

    always trying but not always successful

  10. #10
    Automated Home Guru FrankMcAlinden's Avatar
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    Default Re: RCDs

    Hi Ric
    Had a quick look in the Clipsal Catalogue and they do 1amp circuit breakers 6ka or 10ka .. here in Oz.....So i would imagine Merlin Gerin or others would manufacture 1 amp ones as well.... I still would recommend hrc fuses maybe using fused terminals ;-)
    Frank

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